1. Field of the Invention
The present invention relates to a process for producing a high integrity container having a three-layered wall structure.
2. Description of the Prior Art
With the continuous increase in the amounts of various radioactive wastes generated from nuclear power plants and other nuclear facilities, as well as harmful heavy metal sludges issued from chemical plants, operators and researchers are making every effort to develop safe and economical ways to store and finally dispose of these wastes.
Radioactive substances differ from heavy metals in that individual nuclides have their own half-lives and need to be isolated from the biosphere for limited periods. In the current nuclear fuel cycle that involves nuclear fission, most of the long-lived wastes originate from the spent fuel reprocessing stage. Beat- and gamma-emitting radioisotopes such as .sup.90 Sr and .sup.137 Cs have half-lives of several hundred years, and alpha-emitting transuranics having atomic numbers of 93 or more have estimated half-lives of hundreds of thousands of years. These radioisotopes are typically discharged as high-level radioactive wastes, and most commonly, they are first stored temporarily as liquids, then solidified by suitabel methods, and permanently stored by various engineering techniques, and subsequently disposed of as required. Intermediate and low level wastes, however, are discharged in far greater amounts than high level wastes and it is generally understood that their half-lives are not more than about a hundred years. In other words, ideal containers for surface storage of low and intermediate level radioactive wastes should confine them safely for at least about a hundred years.
Most of the currently used containers for storing and disposing of low and intermediate level radioactive wastes are based on soft steel drums (hereunder simply referred to as metal drums). In actual operations, these wastes are uniformly compacted or solidified with cement, asphalt or plastics in metal drums. The metal drums are simple to use, relatively inexpensive and have been used successfully in many plants for near-term storage, but they corrode in only about 7 years and are not suitable for long-term storage. When the metal drums stored indoors corrode, not only do they become difficult to handle but also they may cause radiation exposures to personnel, and hence radiation contamination of the biosphere. Stainless steel drums are not practical because, for one thing, they are expensive, and for another, in the long run they are gradually corroded by, for example chlorate ion attack. The OECD-NEA (Nuclear Energy Agency) guideline on packages for sea dumping of radioactive wastes recommends the use of a drum that is lined with concrete to provide a double-layered wall. In Japan and European countries, this type of container usually has a concrete lining 5 to 10 cm thick. Such a thick lining reduces the inner capacity of the drum by 35 to 65%, thereby necessitating the use of many drums to solidify radioactive wastes. What is more, the radioisotopes (hereunder sometimes referred to as RI) in the wastes may diffuse in an uncontrolled manner out of a corroded drum.
To cope with the recent shortage in the storage area available at nuclear facilities, the method of solidifying radioactive wastes with asphalt or plastics have recently been developed. This technique is effective to compact radioactive wastes into a smaller volume, but the asphalt or plastics are highly inflammable and are hazardous in a fire. The dangerous nature of this method is more apparent when the metal drum in which the radioactive wastes are solidified with asphalt or plastics is corroded. As a further disadvantage, permanent storage of radioactive wastes is impossible in a small country such as Japan. For economical use of storage areas, the best way is to dispose of radioactive wastes by dumping them in the sea or burying them under the ground when their radioactivity has decreased below a certain level after extended storage. The conventional metal drum based container is apparently not suitable for long-term surface storage or disposal under the ground, and the development of a new type of container that minimizes the reduction in the inner capacity and which remains stable for a prolonged period has been desired.
A container made of polymer-impregnated concrete (hereunder sometimes referred to as PIC), wherein a precast concrete container is impregnated with a monomer (e.g. methyl methacrylate or MMA) that is subsequently polymerized, is known; and it has high strength, long-term durability and can prevent the leaching of radioactive isotopes. But the concrete used does not have much higher impact resistance and is less refractory than concrete. Therefore, to prevent damage that may occur during shipping (e.g. by dropping thereof or other accidental impacts) or in a disaster such as an earthquake or fire, the PIC wall must have a thickness of at least 80 mm, but this again results in a great reduction in the inner capacity of the container.
A container made of steel fiber reinforced polymer impregnated concrete (hereunder sometimes referred to as SFRPIC) is also known. It is fabricated by impregnating a premolded vessel of steel fiber reinforced concrete (hereunder sometimes referred to as SFRC) with a polymerizable monomer which is subsequently polymerized and cured within the concrete. This SFRPIC container is far superior to a container not subjected to impregnation in respect of strength, impact resistance, corrosion resistance, chemical resistance and fire resistance. But as in the case of the PIC container, the SFRPIC version must have a wall thickness of about 50 mm to prevent accidental damage due to fire, dropping or other deleterious factors that may occur during handling. As a result, its inner capacity is too small to be effectively used as a container for surface disposal or as an isotactic container for sea disposal.
For the reasons stated above, it has been long desired in the art to develop a novel container for storage and disposal of radioactive or industrial wastes that is free from the defects of the conventional product.